The present invention concerns a method for making an air-containing mixture, where air is introduced into a viscous mass of raw mixture for forming the air-containing mixture, and where the introduction of air is performed in connection with introducing the mixture in a treating unit, where the raw mixture under pressure is pumped through a nozzle. The invention also concerns an apparatus for making an air-containing mixture, where air is introduced into a viscous mass of raw mixture for forming the air-containing mixture, and where the introduction of air is performed in connection with introducing the mixture in a treating unit, where a mixing tank is provided between a pipe for introducing the mixture and the treatment unit.
The present invention has appeared in connection with working with mixtures in the form of ice mixtures for treatment in a treatment unit in the shape of an ice cream freezer. However, the general principle of the invention may be used in connection with other forms of mixing, where there is a need for admixing air in a viscous unit before introducing in a treatment unit.
A continuous ice cream freezer is in principle a scraper heat exchanger in which a fluid mass (called ice mixture) is cooled and partly frozen at the same time as stirring and admixing of air are performed, often in almost as large amounts as the fluid mass so that the finished product (ice cream) is a highly viscous foam.
The scraper heat exchanger is most frequently designed as a pipe (called a freezer tube) which is cooled externally and with a rotating shaft (called scraper shaft) inside. On the rotating shaft is mounted a number of knives that scrape the frozen ice off the pipe as well as producing a certain mixing action due to their movement through the product. The shaft itself may be designed in different ways; sometimes just as a fixed shaft, at other times with a built-in eccentric shaft, possibly with rotors mounted thereon which assist the admixing process.
Normally, the air is supplied through a check valve in immediate vicinity of the pipe where the fluid mass is introduced in the freezer tube.
The admixing of air then occurs exclusively by means of the movement of the scraper shaft through the mass (and thereby the movement of the knives and the eccentric/the rotors).
The entire production process of the ice cream normally occurs under pressure (typically 3-4 bars) as the ice mixture is pumped into the freezer tube by means of a mixing pump., and the ice cream leaves the freezer tube in a controlled way, either by means of a pump or by a suitable restrictor that ensures a correct pressure in the freezer tube itself.
Depending on which type of ice cream to be produced by the freezer, it may be advantageous to operate with different rotational speeds of the scraper shaft. For a very fluid product with high air content, a high speed is required to get the air “whipped” into the product. By a very firm, extrudable product, a lower speed is better in order not to add heat in the form of mechanical energy to the product.
At a very low rotational speed, such as required by a very firm ice cream, it may be difficult to achieve a sufficient mixing of ice mixture and air, whereby there is a risk of variations in the density of the product, large air bubbles and the like. Today, this is often solved by mounting a motorized pre-whipper at the inlet of the freezer tube. The pre-whipper is normally comprised by a number of stators and a number of rotors with small mutual spacing, where the rotors are driven by an electric motor.
Air and ice mixture are supplied together at the inlet of the pre-whipper, and when passing through the pre-whipper the air bubbles are broken due to the intensive movement, so that the freezer is thus receiving a ready-whipped foam for cooling/freezing.
The greatest drawbacks of a pre-whipper is the price and the increased maintenance of bearings and sealings in it.